The present invention relates, in general, to semiconductor devices and, more particularly, to power semiconductor devices.
Ignition systems typically include power semiconductor devices such as power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) to generate high voltages at the spark plug of the ignition system. In general, the power MOSFET is coupled to the spark plug via a transformer. The drain of the power MOSFET is connected to the primary coil of the transformer and the secondary coil of the transformer is connected to the spark plug. In addition, the primary coil is coupled for receiving a supply voltage or a battery voltage.
When the power MOSFET is on, the voltage at the secondary coil is a predicted value equal to approximately the battery voltage multiplied by the turns ratio of the transformer. When the power MOSFET is switched on and then switched off, the energy stored in the transformer causes the drain voltage of the power MOSFET to rise substantially higher than the battery voltage. This creates an increase in the voltage at the primary coil of the transformer that results in an increase in the voltage at the secondary coil of the transformer. A spark is produced by the spark plug when the voltage at the secondary coil increases to a level higher than a threshold voltage of the spark plug.
A potential problem known as "spark-on-make" can occur when the power MOSFET is initially turned on. "Spark-on-make" is caused by the voltage drop at the drain of the power MOSFET resonating because of the leakage inductance of the transformer, the parasitic capacitance of the transformer, and the parasitic capacitance of the spark plug to produce a voltage at the secondary coil that exceeds the threshold voltage of the spark plug. The resonating has a ringing effect or oscillation where the voltage at the secondary coil overshoots the predicted value of voltage at the secondary coil. This overshoot voltage can generate an undesirable spark when the power MOSFET is initially turned on.
Previously, a technique for preventing "spark-on-make" in an ignition system included adding a blocking diode in series with the spark plug. Problems with this technique include the cost of the blocking diode, the reliability of the blocking diode, and the added space for the blocking diode. Another technique used to prevent "spark-on-make" included reducing the turns ratio of the transformer. A disadvantage of this technique is that a higher voltage at the primary coil is needed to generate the threshold voltage of the spark plug at the secondary coil. Therefore, a higher voltage power semiconductor device requiring more die area is typically used to generate the higher voltage at the primary coil.
Accordingly, it would be advantageous to have a power semiconductor device and a method for preventing "spark-on-make". It would be of further advantage for the power semiconductor device to be cost efficient and reliable.